The US Army wants a means of determining if mysterious objects may be improvised bombs. Specifically, they want to be able to fire a chemical pellet at an object and tell from the color of the splatter if the object is an explosive:

The system would work by loading up projectiles with materials that advertise the presence of explosives — sort of like a litmus test for bombs — and firing them at the suspected bombs. Picture paintballing, but with a target that might really kill you. [...]

The paintball idea is comparatively low-tech. The Army notes that the technology to detect explosives with paints and powders is already a commercial reality. They point to Raptor Detection Technology’s SAFE-T Spray, which turns orange on contact with certain explosives, as an example.